Plastic films such as Polyethylene terephthalate (PET), polypropylene (PP) and polyethylene (PE) are extensively used in packaging due to low cost, strength and stiffness, transparent and flexibility properties. In packaging, specifically for food packaging applications, a good barrier layer against oxygen and moisture is important to prolong the shelf life of packaged food. Oxygen and moisture that transmit from surrounding environment into the packaging will accelerate the oxidation and spoilage of food and encourage rapid mold growth during storage and transportation periods. Despite having a good mechanical strength and moldability properties, the barrier performance of commercially available plastic films in the market today is still relatively poor.
Polymer composites have been developed mainly to improve the mechanical properties and barrier performance of polymers to moisture and gases such as oxygen and carbon dioxide. Polymer composites are mixtures of polymers with inorganic or organic fillers with certain geometries. Inorganic fillers with a high aspect ratio such as nanoclay are particularly interesting because of their high specific surface area, availability, low cost, significant reinforcing effects and simple processability. According to Nielsen's theory (1967), the presence of nanoclay fillers in polymer matrix increases the tortuosity of the diffusive path for a penetrate molecule, thus providing an excellent barrier property. In recent years, many research works have been conducted on the modification of clay surfaces with organic compounds such as silanes with the aim to maximize the barrier performance by improving nanoclay compatibility with polymers. Besides the barrier performance, mechanical integrity of composite film and its bonding strength to plastic substrate are further factors affecting composite packaging performance.
The preparation methods and coating solutions for making composite films known from the prior art preferably make use of silane modified clay composites. However, the preparation methods for such modified clay are not satisfying. As known from the prior art, it is generally difficult to dissolve or disperse silanes in aqueous clay suspension because of their organophilic nature. To overcome this problem, previous approaches have used additives such as surfactants to form silane emulsions prior to the reaction with an aqueous clay suspension. The silane is first emulsified into water containing surfactant, which acts as a wetting agent and emulsifier. Then, the emulsified silanes are mixed with the clay silicates. Accordingly, the surfactant used in this case is limited to a specific range of hydrophilic/lipophilic balance values. In addition, the surfactant concentration and pH at which the silane/surfactant emulsion is prepared are very important to the resulting silane emulsion stability. By using this method, it is very difficult to mix different types of silanes into one pot of aqueous clay suspension in a one-step process, because there a reaction may occur between the two silanes before reacting with clay silicates, which will result in reduced surface treatment of clay and bonding strength to the polymer matrix. Moreover, the presence of residual surfactants in the finished silane modified clay product may influence the properties of barrier layer even though its concentration is low. All these parameters limit the efficacy of the process and productivity in their preparation.
Another known method is directed to dissolving or dispersing silanes into aqueous clay suspension via a solvent exchange technique. In this method, the clay silicate aqueous suspension may be subjected to a solvent exchange whereby the solvent is capable of dissolving the silane compound. The solvent may be an organic solvent selected from methanol, ethanol, propanol, butanol, or pentanol or ketone, propanone or 2-butanone. The solvent exchange method involves washing and filtration steps. After silane-clay reaction, the solvent is removed and the silane modified clay is redispersed in aqueous solution. This modification process is cumbersome and may increase manufacturing cost. It further involves the use of organic solvent which makes it less environmentally acceptable.
In summary, it is found that the barrier films known from the prior art are not fully satisfying with regard to bonding strength to plastic films or do not show the desired high transparency. Others are very cumbersome to prepare and require the use of emulsifier/surfactant or organic solvent during the preparation.
Therefore, there is still a need to provide more cost effective, time saving and environmentally friendly methods to produce clay/polymer composite films with good oxygen and moisture barrier property combined with good bonding strength that overcome, or at least ameliorate, one or more of the disadvantages described above.